The present invention relates to apparatuses and methods for quickly setting an automatic substrate centering system in a semiconductor wafer processing machine to the correct position for centering substrates.
The production of semiconductor wafers and integrated circuits fabricated on such wafers involve many processing steps utilizing many different machines. As wafers are transferred onto each machine, care must be taken to ensure that each wafer is properly positioned on the machine so the wafer will be processed correctly. In particular, machines that use rotatable vacuum chucks to spin wafers require the wafers to be positioned with great precision. A rotatable vacuum chuck is a device that rotates a wafer, sometimes at very high speeds, while holding the wafer firmly in place with a vacuum. The chuck includes a surface on which a wafer may be placed, and a vacuum port extending through the surface. The surface is designed to provide an airtight seal when in contact with a wafer. Thus, when a wafer is placed on the surface, a vacuum can be created underneath the wafer, causing the wafer to be pressed firmly against the surface of the chuck by the air pressure differential across the wafer.
Rotatable vacuum chucks are used in many processes in the semiconductor industry. For instance, in the production of integrated circuits, they are used to apply photoresist to a wafer. In this process, a wafer is first mounted to a chuck. A solution of photoresist is then deposited on the surface of the wafer, either while the wafer is stationary or while it is spinning slowly. Once the photoresist solution has been applied, the wafer is spun at a much higher rate, typically 3,000 to 7,000 rpm, to remove excess photoresist from the wafer surface and to evaporate the solvent, drying the photoresist on the wafer in a thin, uniform layer.
A similar process is used in the production of semiconductor wafers to prepare a wafer for a polishing step. Toward the end of the semiconductor wafer production process, a polishing process is used to remove any damage to the wafer surfaces caused by prior production steps. The polishing process is generally a batch process, where several wafers are mounted to a single glass plate and polished at the same time. The wafers are attached to the glass plate with a thin layer of wax that is applied to the back of each wafer through a spin-coating process that uses a vacuum chuck. In some automated systems for mounting wafers to a glass polishing plate, the glass plate on which the wafers are mounted is rotated between individual mounting positions on a rotatable vacuum chuck, and the glass plate must be centered on the chuck. Rotatable vacuum chucks are also used in some drying processes, and in other machines such as some rapid anneal furnaces.
In each of the above processes, problems may arise if the wafer or glass plate is not positioned in a centered location on the chuck. For instance, in the wax spin-coating process, if the wafer is not centered on the chuck, the resulting wax coating may not have a uniform thickness across the surface of the wafer. Some parts of the wafer may not be coated with wax at all. If a wafer with such an uneven wax coating is mounted to a glass plate for polishing, the surface of the wafer may not be parallel to the surface of the polishing pad, and the polishing process may damage the wafer. At the very least, the polishing may introduce some thickness variation into the wafer. It is possible that the polishing may even chip or break the wafer, rendering the wafer unsuitable for commercial use. Similarly, in a photoresist coating process, spinning a solution of photoresist onto a non-centered wafer may result in the formation of an uneven layer of photoresist on the wafer. The thinner regions of photoresist may provide inadequate protection to the wafer from such processes as ion implantation, and thicker regions may not cure or develop properly. In either case, this may cause the wafer to be unsuitable for later processing.
Other problems may arise when a wafer, glass plate or other substrate is not correctly centered on a rotatable chuck. If a wafer is mounted too far off-center on a chuck, the moment of inertia of the spinning wafer may be sufficient to overcome the vacuum holding the wafer to the chuck. This may cause the wafer to fly off of the chuck during processing, resulting in damage or contamination to the wafer. Also, if a glass plate on which wafers are mounted for polishing is not correctly positioned on the mounting machine, a wafer may be incorrectly mounted to the glass plate. For proper polishing, the wafers should be mounted no closer than approximately 8 millimeters from the edge of the glass plate, because the surface of the plate may have some curvature within 8 millimeters of the plate edge. If a wafer is mounted in this curved region, the curvature may cause a gap to exist between the wafer and the plate. This gap may then cause the wafer to be damaged during the polishing process. If the glass plate is not correctly centered on the rotating chuck of the wafer-mounting machine, the edge of the glass plate will not be in the same position relative to the mechanical wafer-mounting arm for each wafer mounted on the plate. Thus, there is some danger that a wafer may be mounted too close to the edge of the plate, giving rise to these potential problems when the wafer is polished.
To ensure that a wafer is correctly positioned on a wafer processing apparatus, some machines have a substrate centering system. Several types of substrate centering systems are commonly used. One type, used with some rotatable vacuum chucks, consists of two or more plate-like pieces mounted to the machine in the same plane as a wafer positioned on the chuck. The pieces have half-moon shaped recesses formed in the edges adjacent the wafer, and are designed to converge upon the wafer so that the wafer is enclosed within the recesses and pushed to a centered position. Another system uses a four-point centering mechanism, in which four ram-like contacts converge on a wafer or other substrate at ninety-degree angles to each other to push the wafer into a centered position. Other processing machines may not have such a contact centering system, but instead may rely on the robotic mechanisms that transfer a substrate or wafer to the machine to place the substrate or wafer in a centered position.
All of above centering mechanisms rely on some mechanical mechanism to center a wafer. However, the mechanical mechanisms themselves must be adjusted to the correct centering position before they will center wafers correctly. The centering system adjustment process can be time consuming, often requiring 3-6 hours of machine down time to complete. Also, the adjustment process must be repeated whenever a machine is brought down for routine maintenance or somehow knocked out of adjustment.
To adjust a centering system, a wafer or substrate generally must first be manually centered on the rotatable chuck. This is typically done visually. First, an ordinary wafer or substrate is placed in an estimated centered position. Wafers generally have a diameter of 6 or 8 inches, and glass polishing plates are even larger. In contrast, the chuck surface typically has a diameter of around 1.5 inches. Visually inspecting the centering is difficult, as the wafer or substrate generally obscures the chuck. It is thus generally necessary to use a ruler to measure the distance from the edge of the chuck to the edge of the wafer all the way around the chuck. Next, the vacuum is turned on and the chuck is spun to check for any wobble indicating that the wafer or substrate is off-center. If wobble is detected, the process is repeated in a trial-and-error fashion until no wobble is detected in the spinning wafer or substrate. After the wafer or substrate has been centered, the centering system is brought into contact with the wafer or substrate, and is then set to place or push other wafers or substrates to the same position.
Because of the tedious nature of this trial-and-error process, wafer centering tools have been developed. Some of these centering tools have a flat bottom surface with a downwardly extending collar that is configured to precisely fit around the outside of the chuck so that the tool is automatically centered when it is placed on the chuck, as shown in U.S. Pat. No. 4,659,094. These tools also have an upper portion with a wafer-shaped extension around their diameter so that the wafer centering system may be set to the correct position by contacting it with the extension. This design has problems, however, in that it is not adaptable for use on chucks of different shapes or sizes. A different tool must be used for each wafer or substrate size and for each chuck. Thus, it would be desirable to have an improved method and apparatus for quickly setting a substrate centering system to center wafers or substrates in a semiconductor wafer processing machine that may be used on chucks of different shapes and sizes.
One aspect of the present invention provides an apparatus for use in adjusting a substrate centering system to center a substrate on a rotatable chuck in a semiconductor processing machine, the chuck including at least one reference point. The apparatus comprises a plate configured to be placed on the chuck and at least one centering mark formed on the plate, wherein the at least one centering mark is configured so that it may be compared to the at least one reference point on the chuck to determine if the plate is centered. The plate may be at least partially transparent so that its position relative to the at least one reference point on the chuck may be easily seen.
Another aspect of the present invention provides a method of adjusting a substrate centering system to center a substrate on a rotatable chuck in a semiconductor wafer processing machine, the chuck including at least one reference point. The method comprises (1) providing a centering tool to be centered on the chuck, wherein the centering tool includes at least one centering mark; (2) placing the centering tool on the chuck; (3) comparing the at least one reference point on the chuck to the at least one centering mark on the centering tool; (4) moving the centering tool to position the at least one centering mark in a predetermined position relative to the at least one reference point on the chuck so that the centering tool is centered on the chuck; and (5) using the centering tool to adjust the substrate centering system to center a substrate on the chuck.
Yet another aspect of the present invention comprises an apparatus for use in adjusting a substrate centering system to center a substrate on a rotatable chuck in a semiconductor wafer processing machine, the chuck having a wafer mounting surface with an aperture and the aperture having sides. The apparatus comprises a centering tool configured to be centered on the chuck, the centering tool having a lower surface; and a structure extending downwardly from the lower surface of the centering tool, wherein the structure is configured to fit within the aperture in the wafer mounting surface of the chuck to center the centering tool on the chuck.
Another aspect of the present invention provides a method of adjusting a robotic arm to place a wafer on a transfer arm in such a position that the transfer arm will place the wafer in a centered position on a chuck in a wafer processing machine, the chuck including at least one reference point. The method comprises (1) providing a transparent circular disk of the same diameter as the wafer to be transferred to the chuck, wherein the disk has a center, and wherein at least one circle is formed on the disk such that the center of the at least one circle is the center of the disk; (2) placing the disk on the chuck; (3) comparing the at least one reference point on the chuck to the at least one circle on the disk; (4) moving the disk to position the at least one circle in a predetermined position relative to the at least one reference point on the chuck so that the disk is centered on the chuck; (5) positioning the transfer arm in a first home position in contact with the disk on the chuck so that the disk is in an optimized location on the transfer arm; (6) marking the optimized location of the disk on the transfer arm; (7) moving the transfer arm to a second home position adjacent the robotic arm; and (8) adjusting the robotic arm so that the robotic arm transfers a wafer to the transfer arm at the marked optimized location on the transfer arm.